1. Field of the Invention
The present invention relates to a light emitting device using an element which has a light emitting material sandwiched between two electrodes (hereafter referred to as “light emitting elements”). Specifically, the present invention relates to a technique for making a light emitting device with a narrower frame.
2. Description of the Related Art
The development of a light emitting device using a light emitting element has advanced in recent years. The light emitting device does not require a backlight such as used in a liquid crystal display since the light emitting element has the ability of self-light emission. It therefore becomes possible to make light emitting devices thinner and lighter. In addition, the light emitting devices can have a wide angle of view, and therefore they are suitable for outdoor use.
There are two types of light emitting devices, a passive type (simple matrix type) and an active type (active matrix type), and the development of both types is flourishing. In particular, active matrix light emitting devices are in the spotlight at present. Further, there are organic and inorganic materials for the materials that are used in light emitting layers of light emitting elements. Furthermore, organic materials are divided into low molecular weight organic materials and high molecular weight (polymer) organic materials, and both types are being researched at a feverish pace. Low molecular weight organic materials are formed mainly by vacuum evaporation, and high molecular weight organic materials are mainly formed by a method such as spin coating or ink jet printing.
Compared to inorganic materials, organic materials have high light emission efficiency, and can be driven at a low voltage. Further, in organic compounds, it is possible to design and make various types of new substances. Thus, there is a possibility of discovering an element that emits light at higher efficiency in accordance with future developments in material design.
An example of a light emitting device in which seepage of a sealing material has developed is shown in FIGS. 20A and 20B. FIG. 20A is a cross sectional diagram of a device in which seepage of a sealing material has developed, cut along a dotted line segment A—A′, and FIG. 20B is an upper surface diagram of the device. A light emitting element 1208 is composed of an anode 1202, an organic layer 1203, and a cathode 1204, and is formed so that the organic layer 1203 is sandwiched by the anode 1202 and the cathode 1204. Although the anode or the cathode may be formed directly on a first substrate 1201, the anode is in general formed on the first substrate 1201 due to ease of manufacture. Electrons injected from the cathode and holes injected from the anode recombine at light emitting centers of the organic layer 1203 to form excitons, and the excitons emit energy for emitting light in returning to a base state. Further, a wiring portion 1209 and a driver circuit 1212 are formed on the first substrate 1201. The wiring portion is a gathering of wirings that are electrically connected to the driver circuit. Although not shown in the figures, the driver circuit 1212 and the light emitting element 1208 are also electrically connected by wirings.
Sealing of the light emitting element is described next. At present, a method of sealing by using a sealing can or a glass substrate has been devised. For present-day product panels, a drying agent is placed into a metallic sealing can, an adhesive material is applied on the periphery of the sealing can, and the adhesive is hardened, to perform sealing (refer to “Organic EL and Display”, CMC, p. 250, 2001). The sealing can and the glass substrate are both referred to as a second substrate 1200. Further, the adhesive material corresponds to a sealing material. That is, the sealing material is applied to the second substrate 1200, and the first substrate 1201 and the second substrate 1200 are bonded through a seal pattern 1205. The sealing material bonds the first substrate and the second substrate, is a bonding and sealing material for enclosing the light emitting elements therebetween, and is disposed in a peripheral portion of the panel. The seal pattern is regulated by the shape, position, and width of the sealing material. The light emitting element 1208 is in a sealed space surrounded by the first substrate 1201, the second substrate 1200, the seal pattern 1205, and the driver circuit 1212. Light emitting elements deteriorate due to moisture and oxygen, and therefore the sealed space is filled with an inert gas 1206 (molecular nitrogen or a noble gas). In this specification, a region surrounded by the first substrate, the second substrate, the seal pattern, and the light emitting element is referred to as a closed space. In addition, in this specification, the first substrate refers to the substrate on which the light emitting element is formed, and the second substrate refers to the substrate bonded to the first substrate through the seal pattern in order to protect the light emitting element, which easily deteriorates due to moisture and oxygen. A drying agent is omitted from being shown in the figures.
As shown in FIGS. 20A and 20B, the light emitting device has structure in which the first substrate 1201 on which the light emitting element is formed, and the second substrate 1200 are bonded through the seal pattern 1205. The sealing material is pushed down upon and spreads out after bonding the second substrate 1200, on which the seal pattern 1205 is formed by applying the sealing material, and the first substrate 1201, and the width of the seal pattern 1205 becomes greater. The term “display portion” corresponds to a region of the light emitting element as seen from a normal direction to the first substrate in this specification. There has been a case in which there occurs expansion of a seal pattern (seepage of a sealing material) 1207 from an area where the seal pattern is to be formed and the seal pattern leaks out onto the display portion of a display or onto edge faces of the substrate.
The following can be considered to be causes of seepage of the sealing material in liquid crystal display devices. The temperature of the panel is generally increased to a temperature between 150 and 200° C. by a thermal pressing after the process of bonding the first substrate and the second substrate to proceed a hardening reaction, and the temperature of the panel is reduced to a room temperature when hardening is complete. The viscosity of the sealing material becomes lower at a temperature of 150 to 200° C. immediately before the hardening reaction begins, and the sealing material is in a very fluid state. The sealing material therefore flows out through any portions in which even a slight gap between the first substrate and the second substrate exists due to capillary action. In addition, the hardening reaction begins as time elapses, therefore the viscosity of the sealing material increases rapidly, and the sealing material that has once spread out will not return to its prior state, instead to harden as is.
It may be difficult to perform sectioning a portion of the first substrate or the second substrate near the seal pattern formed between the first substrate and the second substrate. This interferes with the technique of making the light emitting device into a light emitting device with a narrower frame. Making a narrower frame means to make the distance between the light emitting element 1208 and an edge face 1211 smaller. Further, regulation of the amount of the sealing material applied can be considered for simply making the width of the seal pattern narrower in order to achieve a narrower frame of a display. However, making the width of the seal pattern thinner can also cause a reduction in the bond strength between the first substrate and the second substrate, as well as seal peeling. In order to perform precise control of the amount of the sealing material applied, a high cost dispenser apparatus (seal drawing apparatus) for performing control of the substrate gap is required.
In recent years, small size displays have been in demand for portable devices such as portable telephones while high definition is desired in order to display dynamic images. It is necessary to make the resolution higher (the number of pixels becomes greater), and to reduce the pixel pitch in order to have higher definition. However, a pixel pitch may be on the order of 300 μm, for example, in displaying a character, and therefore, the number of pixels must be increased in order to have higher definition, which requires to increase the proportion occupied by the display portion in the portable device. Making a narrower frame is thus a significant object.